The technology of forming a conductive circuit by printing an ink composition containing conductive particles is already commercially implemented, for example, in the solar cell application wherein a conductive circuit is formed on a cell substrate by screen printing. A number of improvements in this technology have been proposed. For example, Patent Document 1 discloses that a conductive ink composition containing metal particles and glass frit, which is commonly used in the art, is printed by screen printing with the aid of ultrasonic oscillation. This method enables high speed formation of conductive circuits.
A problem arises when a circuit is formed on a semiconductor circuit board using a conductive ink composition based on glass. If the board is heated after circuit formation for substrate bonding or packaging purpose, the conductive ink composition may be cracked or otherwise stressed to cause a resistance change or breakage to the conductor. There is a need for a circuit-forming material having high stress resistance. Silicone material is characterized by heat resistance and stress relaxation. Patent Document 2 discloses that an ink composition comprising a thermoplastic resin, epoxy-modified silicone, metal powder, and silicone rubber elastomer is diluted with a solvent and used to form a conductive circuit which is not cracked or adversely affected on heat treatment. It is also known that conductive particles are dispersed in silicone rubber to form an ink composition.
The current trend is toward miniaturization of semiconductor circuits, and the size of concomitant conductive circuits also becomes finer. Also efforts are made on the so-called 3D semiconductor device, that is, a stacked semiconductor circuit structure obtained by forming a semiconductor circuit on a substrate, and stacking two or more such substrates. When such fine semiconductor circuits are provided with a plurality of contacts and packaged, or when interconnects are formed between semiconductor circuits on two or more silicon substrates, the conductive circuit to be connected is not only required to be resistant to thermal stress, but also needs to control its shape as fine structure.
For instance, if a conductive circuit including lines of different width is formed using a conductive ink composition containing a solvent, the flatness or shape of conductor lines may change in some areas before and after curing, or a height difference of the circuit may develop under the influence of certain factors such as the volatilization rate of the solvent. If connection is achieved while taking into account the influence, the margin for miniaturization may be lost. In attempts to achieve further miniaturization of semiconductor devices or to construct 3D stacking of semiconductor devices, it would be desirable to have a technology of forming a conductive circuit using a conductive ink composition that allows for stricter control of the circuit shape.
The above-mentioned ink composition having metal particles dispersed in silicone rubber becomes useful in forming a conductive circuit by printing when a thixotropic agent is added thereto. The printed circuit maintains its shape unchanged before and after curing. Further the circuit thus formed has a high stress relaxation ability to thermal stress or the like. However, when only conductive particles having a particle size of at least 5 μm contributing to least thixotropy are used, a problem arises with regard to conduction to electrodes. Although the resistivity of the bulk can be controlled in accordance with the amount of conductive particles added, there are available least conductive paths between conductive particles in the silicone rubber composition and electrodes, which may lead to a high contact resistance. An improvement in this respect is desired.